Amorphous silicon-carbon composite, preparation method therefor, and lithium secondary battery comprising same

ABSTRACT

An amorphous silicon-carbon composite, a method for preparing the amorphous silicon-carbon composite using a pyrolysis method, a negative electrode for a lithium secondary battery, and a lithium secondary battery including the same.

TECHNICAL FIELD

This application claims priorities to and benefits of Korean PatentApplication No. 10-2018-0029924, filed on Mar. 14, 2018 and KoreanPatent Application No. 10-2019-0026971, filed on Mar. 8, 2019, theentire disclosure of which are incorporated herein by reference itsentirety.

The present invention relates to an amorphous silicon-carbon composite,a method of manufacturing the same, a negative electrode for a lithiumsecondary battery and a lithium secondary battery comprising the same.

BACKGROUND ART

A lithium secondary battery (for example, a lithium ion battery), anickel metal hydride battery and other secondary batteries are becomingincreasingly important as a power source for a vehicle or a portableremote terminal such as a notebook computer. In particular, the lithiumsecondary battery, which can achieve high energy density with lightweight, can be used as a high-energy power supply for in-vehicle use,thereby being expected to continue to increase in demand in the future.

The lithium secondary battery is manufactured by using a materialcapable of intercalating/deintercalating lithium ions as an activematerial of a negative electrode, installing a porous separator betweenthe positive electrode and a negative electrode, and injecting a liquidelectrolyte, wherein electricity is generated or consumed by the redoxreaction resulting from the intercalation and deintercalation of lithiumions at the negative electrode and the positive electrode.

Specifically, in the lithium secondary battery, a variety ofcarbon-based materials including artificial graphite, natural graphite,hard carbon, and soft carbon capable of intercalating anddeintercalating lithium have been applied as a negative electrode activematerial. A battery using graphite among the carbon-based materials as anegative electrode active material not only exhibits a high dischargevoltage of 3.6 V, but also provides an advantage in terms of energydensity of a lithium secondary battery, and is most widely used becauseit guarantees long lifetime of lithium secondary battery due toexcellent reversibility. However, in the case of the graphite activematerial, when manufacturing the electrode plate, the density(theoretical density 2.2 g/cc) of the graphite is low, so that thecapacity is low in terms of the energy density per unit volume of theelectrode plate, and a side reaction with the organic electrolytesolution used is easily generated at a high voltage, and thus there areproblems of generation of gas and as a result, capacity reduction.

In order to solve the problems of such carbon-based negative electrodeactive material, Si-based negative electrode active material with veryhigh capacity compared with graphite has been developed and studied.However, a Si-based negative electrode material with a high capacity isaccompanied by a severe volume change during charging/discharging, andthus there is a drawback that the particle cleavage is resulted and as aresult, the lifetime characteristic is poor. Specifically, negativeelectrode active materials based on silicon or silicon oxides (SiO_(x),0<x<2) capable of alloying and dealloying with lithium ions have avolume expansion up to 300%. During such volumetric expansion andcontraction, the SiO_(x) negative electrode active material isphysically subjected to severe stress and pulverized out. As a result,the existing SEI layer is destroyed and a new interface is formed, whilea new SEI layer is formed. This leads to continuous decomposition of theelectrolyte solution and consumption of lithium ions, therebydeteriorating the cycle characteristics of the battery. Also, duringcontinuous charging/discharging, the conductive structure is destroyedby the volume expansion and contraction of the SiO_(x)-based negativeelectrode active material and the durability of the electrode isdeteriorated, thereby deteriorating the lifetime of the battery.

To solve these problems, a composite containing void capable ofbuffering the volume expansion has been proposed. However, in the caseof a composite including a void structure, there are problems that thecomposite tends to be broken in the rolling process accompanied duringthe manufacturing of the electrode, and it is difficult to manufacturethe electrode with a high loading amount due to its low density.

Also, as a method for solving these problems, nanotube type SiO_(x) hasbeen developed, but there are problems that the manufacturing process isdifficult and commercialization is difficult because of high unit cost.

PRIOR ART DOCUMENT Patent Document

-   Korean Patent No. 10-1612603, CARBON-SILICON COMPLEX, NEGATIVE    ACTIVE MATERIAL FOR SECONDARY BATTERY INCLUDING THE SAME AND METHOD    FOR PREPARING THE SAME.

DISCLOSURE Technical Problem

It is an object of the present invention to provide an amorphoussilicon-carbon composite which has a small volume change duringcharging/discharging of a lithium secondary battery and does not causefragmentation.

It is another object of the present invention to provide a method ofpreparing an amorphous silicon-carbon composite in which themanufacturing process is simple by using the pyrolysis method.

It is another object of the present invention to provide a negativeelectrode for a lithium secondary battery, which can improve theelectrical conductivity and lifetime characteristic of the battery bycomprising the amorphous silicon-carbon composite, and a lithiumsecondary battery.

Technical Solution

In order to achieve the above objects, the present invention provides anamorphous silicon-carbon composite composed of silicon (Si) and carbon(C) mixed at a molecular level wherein the composite has a diameter of10 nm to 1 μm.

In addition, the present invention provides a method for preparing anamorphous silicon-carbon composite comprising the steps of:

-   -   a) mixing a silane compound containing hydrocarbon with an        organic solvent to prepare a mixed solution; and    -   b) pyrolyzing the mixed solution in an inert atmosphere and        depositing it on a substrate.

In addition, the present invention provides a negative electrode for alithium secondary battery comprising an active material; a conductivematerial; and a binder wherein the active material comprises theamorphous silicon-carbon composite of the present invention.

In addition, the present invention provides a lithium secondary batterycomprising a positive electrode; a negative electrode; a separatorinterposed between the positive electrode and the negative electrode;and an electrolyte solution wherein the negative electrode is thenegative electrode of the present invention.

Advantageous Effects

The amorphous silicon-carbon composite of the present invention has anadvantage that silicon and carbon are mixed in a molecule unit, so thatvolume change during charging/discharging of the battery is small andfragmentation does not occur.

In addition, the method for preparing the amorphous silicon-carboncomposite of the present invention has an advantage that the process issimple.

In addition, the lithium secondary battery comprising the amorphoussilicon-carbon composite of the present invention has excellentelectrical conductivity and lifetime characteristics.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of an amorphous silicon-carbon compositeof the present invention.

FIG. 2 is a transmission electron microscope (TEM) photograph of theamorphous silicon-carbon composite prepared in Example 1-1.

FIG. 3 is a photograph of the amorphous silicon-carbon composite (Si—C)prepared in Example 1-1 measured by using a transmission electronmicroscopy (TEM)—energy dispersive spectroscopy.

FIG. 4 is a scanning electron microscope (SEM) photograph of thesilicon-carbon composite (Si-Graphite) prepared in Comparative Example1-1.

FIG. 5 is a scanning electron microscope (SEM) photograph of thesilicon-oxygen-carbon composite (SiOC) prepared in Comparative Example2-1.

FIG. 6 is a scanning electron microscope (SEM) photograph of theamorphous silicon-carbon composite prepared in Comparative Example 3-1.

FIG. 7 is a transmission electron microscope (TEM) photograph of theamorphous silicon-carbon composite prepared in Comparative Example 3-1.

FIG. 8 is an X-ray diffraction graph of Experimental Example 1.

FIG. 9 is an electrical conductivity graph of Experimental Example 2.

FIG. 10 is an initial charging/discharging graph of Example 1-3, andComparative Examples 1-3 and 2-3.

FIG. 11 is an initial charging/discharging graph of Example 1-3 andComparative Example 3-3.

FIG. 12 is a graph of charging/discharging lifetime characteristics ofExample 1-3, and Comparative Examples 1-3 and 2-3.

FIG. 13 is a graph of charging/discharging lifetime characteristics ofExample 1-3 and Comparative Example 3-3.

FIG. 14 is a graph of charging/discharging characteristics according toC-rate of Example 1-3, and Comparative Examples 1-3 and 2-3.

BEST MODE

Hereinafter, the present invention will be described in more detail.

Silicon has about 10 times the capacity of graphite, but has a problemthat volume change and fragmentation occur during charging/dischargingof the battery, thereby deteriorating the lifetime characteristic of thebattery.

Therefore, in order to solve the above problems, there has been proposeda method of simply mixing silicon and carbon or coating carbon on thesurface of silicon and then mixing it with graphite. However, the aboveconventional techniques have not achieved smooth contact between siliconand carbon and thus are not excellent in ion conductivity and electricalconductivity, and also has non-uniform distribution of silicon andcarbon and thus still did not solve the above problem.

Therefore, in order to solve the above problems, the present inventionprovides an amorphous silicon-carbon composite composed of silicon (Si)and carbon (C) mixed at a molecule unit.

The amorphous silicon-carbon composite of the present invention cansolve the above-mentioned problems because silicon and carbon are mixedin a molecule unit.

Amorphous Silicon-Carbon Composite

The present invention relates to an amorphous silicon-carbon compositecomposed of silicon (Si) and carbon (C) mixed at a molecular level,wherein the composite has a diameter of 10 nm to 1 μm.

The amorphous silicon-carbon composite is formed by a pyrolysisdeposition process for a silicon source and a carbon source. Thecomposite comprises a silicon-carbon covalent bond, a silicon-siliconcovalent bond, and a carbon-carbon covalent bond. The covalent bonds areirregularly present in the composite.

In addition, the amorphous silicon-carbon composite may further comprisea heteroatom. In this case, the composite may further comprise at leastone of a heteroatom-carbon covalent bond and a heteroatom-siliconcovalent bond, and the covalent bonds are irregularly present in thecomposite.

The heteroatom may be at least one selected from the group consisting ofboron (B), phosphorus (P), nitrogen (N), and sulfur (S).

In the present invention, the amorphous silicon-carbon composite isformed by a pyrolysis deposition process of a silane compound containinghydrocarbon, and when the compound is pyrolyzed, the amorphoussilicon-carbon composite is formed due to breakage of a part or wholebond of the compound. That is, the silicon source and the carbon sourcemay be silane compounds including hydrocarbon. Accordingly, thecomposite of the present invention has a distribution of silicon andcarbon without a concentration gradient, and thus when applying tolithium secondary battery, can minimize the problem of volume expansion,and improve the lifetime characteristic of the battery.

The amorphous silicon-carbon composite comprises silicon and carbon in aweight ratio of 3:7 to 7:3.

If the amount of silicon is less than the above range, the capacity ofthe battery may be decreased, and if the amount of silicon exceeds theabove range, the lifetime of the battery may be decreased.

Also, if the amount of carbon is less than the above range, the lifetimeof the battery may be decreased, and if the amount of carbon exceeds theabove range, the capacity of the battery may be decreased.

The amorphous silicon-carbon composite may also comprise very smallamounts of hydrogen and oxygen.

The amorphous silicon-carbon composite may be in the form of particles,and the diameter of the composite may be 10 nm to 1 μm, preferably 100to 500 nm. If the diameter of the composite is less than 10 nm, thedensity of the composite is significantly lowered, as well as it isdifficult to manufacture the electrode. If the diameter of the compositeexceeds 1 μm, the electrical conductivity of the composite is greatlyreduced, and the lifetime and rate performance of the battery may bedegraded.

In addition, the amorphous silicon-carbon composite has a density of 0.2to 0.6 g/cc, preferably 0.3 to 0.5 g/cc. If the density is less than 0.2g/cc, the density of the electrode is lowered, that is, the thickness ofthe electrode is increased compared to the same loading amount, so thatthe energy density of the battery is lowered. If the density exceeds 0.6g/cc, the resistance of the electrode increases and the rate performancedecreases.

The amorphous silicon-carbon composite of the present invention is amixture of silicon and carbon at a molecular level, which is composed ofa plurality of silicon atoms, a plurality of carbon atoms and covalentbonds thereof, and can be used as a negative electrode active materialof a lithium secondary battery.

If the amorphous silicon-carbon composite of the present invention isused in a lithium secondary battery, problems such as volume change andfragmentation of silicon occurring during charging/discharging of thebattery can be solved, and the battery exhibits excellent electricalconductivity and lifetime characteristics.

Method for Preparing Amorphous Silicon-Carbon Composite

In addition, the present invention relates to a method for preparing anamorphous silicon-carbon composite comprising the steps of:

-   -   a) mixing a silane compound containing hydrocarbon with an        organic solvent to prepare a mixed solution; and    -   b) pyrolyzing the mixed solution in an inert atmosphere and        depositing it on a substrate.

The step a) is a step of mixing a silane compound containing hydrocarbonwith an organic solvent to prepare a mixed solution.

The hydrocarbon-containing silane compound is a compound containinghydrocarbon as a functional group in the silane structure, and the kindthereof is not particularly limited. In the present invention, thesilane compound may preferably comprises at least one selected from thegroup consisting of tetramethylsilane, dimethylsilane, methylsilane,triethylsilane, phenylsilane and diphenylsilane.

In addition, the silane compound containing hydrocarbon may be acompound further containing a heteroatom.

In the present invention, the type of the compound is not particularlylimited as long as the heteroatom can form a covalent bond with siliconand carbon. The heteroatom may be at least one selected from the groupconsisting of boron (B), phosphorus (P), nitrogen (N), and sulfur (S).

The organic solvent may be used without particular limitation as long asit can dissolve a silane compound containing hydrocarbon. The organicsolvent is preferably an organic solvent which has a boiling point of atleast about 100° C., does not have high viscosity and does not causecarbonization at a temperature of 600° C. or higher. In the presentinvention, the organic solvent may specifically comprises, for example,at least one selected from the group consisting of toluene, benzene,ethylbenzene, xylene, mesitylene, heptane and octane.

The organic solvent is used for dilution in order to complement theboiling point of silane compounds which comprise hydrocarbon having arelatively low boiling point. If the pyrolysis temperature is 800° C. ormore, since pyrolysis of organic solvents can occur together, additionalcarbon can be provided in the amorphous silicon-carbon composite tocontrol the ratio of silicon and carbon.

The mixing of the compound and the organic solvent is preferablyperformed at room temperature for about 10 to 30 minutes.

The step b) is a step of pyrolyzing the mixed solution in an inertatmosphere and depositing it on a substrate.

Specifically, the pyrolysis is performed by a process of providing andbubbling an inert gas into the mixed solution, and the inert atmosphereis preferably an argon (Ar) gas atmosphere.

The pyrolysis temperature is 600 to 900° C. If the pyrolysis temperatureis less than 600° C., pyrolysis of the silane compound containinghydrocarbon cannot be carried out and thus then amorphous silicon-carboncomposite cannot be produced. If the pyrolysis temperature exceeds 900°C., not only direct decomposition of the organic solvent can occur andthus the mixing ratio of silicon and carbon can deviate from the desiredmixing ratio, but also it is difficult to control the content of siliconand carbon.

If the content of hydrogen in the amorphous silicon-carbon composite ishigh, hydrogen gas is generated during the operation of the battery andthus the capacity of the battery is deteriorated. Even if the pyrolysistemperature is within the above temperature range, the higher thetemperature is, the lower the content of hydrogen in the amorphoussilicon-carbon composite can be. Therefore, the pyrolysis temperature ispreferably 700 to 800° C.

In addition, the pyrolysis is performed for 10 minutes to 1 hour,preferably 30 minutes to 1 hour.

The amorphous silicon-carbon composite can be prepared by pyrolyzingcompound or organic solvent contained in the mixed solution to obtain anamorphous silicon-carbon composite in which silicon (Si) and carbon (C)is finally mixed at a molecular level.

More specifically, the amorphous silicon-carbon composites produced bythe pyrolysis are deposited on a substrate, and the step of separatingthe deposited composite may finally produce amorphous silicon-carboncomposites in the form of particles. The method of separating thedeposited composite is not particularly limited in the presentinvention, but a ball-mill process can be preferably used.

The diameter of the deposited composite has a size exceeding 1 μm, andthe diameter of the amorphous silicon-carbon composite having a particleshape separated from the substrate is 10 nm to 1 μm, preferably 100 to500 nm. If the diameter of the composite is less than 10 nm, not onlythe density of the composite is significantly lowered, but also it isdifficult to manufacture the electrode. If the diameter exceeds 1 μm,the electrical conductivity of the composite is greatly reduced, and thelifetime and rate performance of the battery may be deteriorated.

The type of the substrate is not particularly limited in the presentinvention, but preferably silicon or an alumina substrate can be used.

In an embodiment of the present invention, a mixed solution prepared bymixing the compound and an organic solvent at room temperature isprepared. After the substrate is placed in the furnace, an inert gas isflowed into the furnace to make the atmosphere of the furnace become aninert atmosphere and then the temperature is constantly adjusted byheating the furnace. Thereafter, the mixed solution is poured into afurnace to pyrolyze the silane compound containing hydrocarbon and thusproduce an amorphous silicon-carbon composite, and the composite isdeposited on a substrate. An amorphous silicon-carbon composite in theform of particles can be obtained through a process such as ball millprocess of the composite deposited on the substrate.

The amorphous silicon-carbon composite is prepared in the form ofparticles through a process of separating a composite, such as a ballmill process, after substrate deposition. The amorphous silicon-carboncomposite is in the form of particles. The diameter of the amorphoussilicon-carbon composite in the form of particles is 10 nm to 1 μm,preferably 100 nm to 500 nm.

The preparation method of the present invention is a method forpreparing an amorphous silicon-carbon composite through a simplepyrolysis method, which has a merit that the manufacturing process issimple.

Negative Electrode for Lithium Secondary Battery

The present invention relates a negative electrode for a lithiumsecondary battery comprising an active material; a conductive material;and a binder, wherein the active material comprises the amorphoussilicon-carbon composite of the present invention.

Specifically, the negative electrode comprises a negative electrodeactive material formed on a negative electrode current collector, andthe negative electrode active material is an amorphous silicon-carboncomposite prepared according to the present invention.

The negative electrode current collector may be specifically selectedfrom the group consisting of copper, stainless steel, titanium, silver,palladium, nickel, alloys thereof, and combinations thereof. Thestainless steel can be surface-treated with carbon, nickel, titanium orsilver, and the alloy may be an aluminum-cadmium alloy. In addition tothose, a nonconductive polymer the surface of which is treated withsintered carbon, i.e. a conductive material, or a conductive polymer,etc. may be used.

The conductive material is used to further improve the conductivity ofthe electrode active material. The conductive material is notparticularly limited as long as it has electrical conductivity withoutcausing chemical changes in the relevant battery, and for example,graphite such as natural graphite or artificial graphite; carbon blacksuch as carbon black, acetylene black, Ketjen black, channel black,furnace black, lamp black, thermal black; conductive fibers such ascarbon fibers and metal fibers; carbon fluoride; metal powders such asaluminum and nickel powder; conductive whiskers such as zinc oxide andpotassium titanate; conductive metal oxides such as titanium oxide;polyphenylene derivatives may be used.

The binder is used for the bonding of the electrode active material andthe conductive material and for the bonding to the current collector.Non-limiting examples of such binder may be polyvinylidene fluoride(PVDF), polyvinyl alcohol (PVA), polyacrylic acid (PAA), polymethacrylicacid (PMA), polymethyl methacrylate (PMMA), polyacrylamide (PAM),polymethacrylamide, polyacrylonitrile (PAN), polymethacrylonitrile,polyimide (PI), alginic acid, alginate, chitosan, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose,polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene,ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM,styrene-butadiene rubber (SBR), fluorine rubber, various copolymersthereof and the like.

In addition, the negative electrode may further comprise a filler andother additives.

Lithium Secondary Battery

In addition, the present invention relates to a lithium secondarybattery comprising a positive electrode; a negative electrode; aseparator interposed between the positive electrode and the negativeelectrode; and an electrolyte solution, wherein the negative electrodeis the negative electrode of the present invention.

The constitution of the positive electrode, the negative electrode, theseparator and the electrolyte solution of the lithium secondary batteryis not particularly limited in the present invention, and is well knownin the art.

The positive electrode comprises the positive electrode active materialformed on the positive electrode current collector.

The positive electrode current collector is not particularly limited aslong as it has high electrical conductivity without causing chemicalchanges in the relevant battery. For example, stainless steel, aluminum,nickel, titanium, sintered carbon, or aluminum or stainless steel whosesurface is treated with carbon, nickel, titanium, silver or the like maybe used. At this time, the positive electrode current collector may usevarious forms such as a film, sheet, foil, net, porous body, foam,nonwoven fabric and the like, which have fine irregularities on asurface, so as to increase the adhesion to the positive electrode activematerial.

The positive electrode active material constituting the electrode layermay be any positive electrode active material available in the art.Specific examples of such positive electrode active materials may be,but is not limited to, lithium metal; lithium cobalt-based oxides suchas LiCoO₂; Li_(1+x)Mn_(2-x)O₄ (wherein x is from 0 to 0.33), lithiummanganese-based oxides such as LiMnO₃, LiMn₂O₃, LiMnO₂; lithiumcopper-based oxide such as Li₂CuO₂; vanadium-based oxide such as LiV₃O₈,V₂O₅, Cu₂V₂O₇; lithium nickel-based oxide represented byLiNi_(1-x)M_(x)O₂ (wherein, M=Co, Mn, Al, Cu, Fe, Mg, B or Ga, x=0.01 to0.3); lithium manganese composite oxide represented by LiMn_(2-x)M_(x)O₂(wherein, M=Co, Ni, Fe, Cr, Zn or Ta, x=0.01 to 0.1) or Li₂Mn₃MO₈(therein, M=Fe, Co, Ni, Cu or Zn); lithium-nickel-manganese-cobalt-basedoxides represented by Li(Ni_(a)Co_(b)Mn_(c)) O₂ (wherein, 0<a<1, 0<b<1,0<c<1, a+b+c=1); sulfur or disulfide compound; phosphates such asLiFePO₄, LiMnPO₄, LiCoPO₄, LiNiPO₄; Fe₂(MoO₄)₃ or the like.

At this time, the electrode layer may further comprise a binder, aconductive material, a filler, and other additives in addition to thepositive electrode active material. The binder and the conductivematerial are the same as those described above for the negativeelectrode for the lithium secondary battery.

The separator may be formed of a porous substrate. The porous substratemay be any porous substrate commonly used in an electrochemical device.For example, a polyolefin-based porous film or a nonwoven fabric may beused, but it is not particularly limited thereto.

The separator may be at least one selected from the group consisting ofpolyethylene, polypropylene, polybutylene, polypentene, polyethyleneterephthalate, polybutylene terephthalate, polyester, polyacetal,polyamide, polycarbonate, polyimide, polyetheretherketone, polyethersulfone, polyphenylene oxide, polyphenylenesulfide, and polyethylenenaphthalate or may be a porous substrate composed of a mixture of two ormore thereof.

The electrolyte solution of the lithium secondary battery is anon-aqueous electrolyte solution containing a lithium salt which iscomposed of the lithium salt and a solvent. The solvent is a non-aqueousorganic solvent, an organic solid electrolyte, and an inorganic solidelectrolyte.

The lithium salt is a substance which is favorably dissolved in thenon-aqueous electrolyte solution, and may be, for example, LiCl, LiBr,LiI, LiClO₄, LiBF₄, LiB₁₀Cl₁₀, LiPF₆, LiAsF₆, LiSbF₆, LiAlCl₄, LiSCN,LiC₄BO₈, LiCF₃CO₂, LiCH₃SO₃, LiCF₃SO₃, LiN(SO₂CF₃)₂, LiN(SO₂C₂F₅)₂,LiC₄F₉SO₃, LiC(CF₃SO₂)₃, (CF₃SO₂)₂·NLi, lithium chloroborane, lithiumlower aliphatic carboxylate, 4-phenyl lithium borate, or lithium imide,etc.

The non-aqueous organic solvent may be, for example, aprotic organicsolvents such as N-methyl-2-pyrrolidone, propylene carbonate, ethylenecarbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate,ethyl methyl carbonate, gamma-butyrolactone, 1,2-dimethoxyethane,1,2-diethoxyethane, tetrahydroxy franc, 2-methyl tetrahydrofuran,dimethylsulfoxide, 1,3-dioxolane, 4-methyl-1,3-dioxen, diethylether,formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane,methyl formate, methyl acetate, phosphate triester, trimethoxy methane,dioxolane derivatives, sulfolane, methyl sulfolane,1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives,tetrahydrofuran derivatives, ether, methyl propionate, and ethylpropionate.

As the organic solid electrolyte, for example, polyethylene derivatives,polyethylene oxide derivatives, polypropylene oxide derivatives,phosphate ester polymers, poly agitation lysine, polyester sulfide,polyvinyl alcohol, polyvinylidene fluoride, and a polymer containingsecondary dissociation group and the like can be used.

As the inorganic solid electrolyte, for example, nitrides, halides,sulfates and the like of Li such as Li₃N, LiI, Li₅NI₂, Li₃N—LiI—LiOH,LiSiO₄, LiSiO₄—LiI—LiOH, Li₂SiS₃, Li₄SiO₄, Li₄SiO₄—LiI—LiOH,Li₃PO₄—Li₂S—SiS₂ may be used.

The non-aqueous electrolyte solution may further contain other additivesfor the purpose of improving charge-discharge characteristics, flameretardancy, and the like. Examples of such additives may be pyridine,triethylphosphite, triethanolamine, cyclic ether, ethylene diamine,n-glyme, hexaphosphoric triamide, nitrobenzene derivatives, sulfur,quinone imine dyes, N-substituted oxazolidinone, N,N-substitutedimidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole,2-methoxy ethanol, aluminum trichloride, fluoro-ethylene carbonate(FEC), propene sultone (PRS), vinylene carbonate (VC).

The lithium secondary battery according to the present invention can bemanufactured by lamination, stacking and folding processes of theseparator and the electrodes, in addition to the usual winding process.The battery case may be a cylindrical shape, a square shape, a pouchshape, a coin shape or the like.

Hereinafter, in order to facilitate understanding of the presentinvention, preferred examples are presented, but the following examplesare intended to illustrate the invention only. It will be apparent tothose skilled in the art that various changes and modifications can bemade within the scope and spirit of the present invention, and also itis obvious that such changes and modifications fall within the scope ofthe appended claims.

Example 1

1-1. Preparation of Amorphous Silicon-Carbon Composite

10 mL of tetramethylsilane (TMS) was dissolved in 10 mL of toluene andmixed at room temperature for 10 to 30 minutes to prepare a mixedsolution.

The furnace was filled with a silicon wafer to be deposited with anamorphous silicon-carbon composite, and then argon (Ar) gas (purity99.999%) was flowed at a rate of 500 cc/min, and thus made the inside ofthe furnace became an inert atmosphere. Thereafter, the furnace washeated to a temperature of 750° C. at a heating rate of 10° C./min.After the temperature of the furnace reached 750° C., the temperaturewas maintained for 10 to 30 minutes to set the temperature inside thefurnace to a constant value.

Thereafter, the mixed solution was poured into the furnace at a rate of100 cc/min and bubbled with argon gas to pyrolyze the mixed solution.

After the pyrolysis, the temperature of the furnace is lowered to roomtemperature, and an amorphous silicon-carbon composites pyrolyzed on thesubstrate in the furnace were obtained. Through a ball mill process ofthe obtained composite, an amorphous silicon-carbon composite (Si—C) inwhich silicon (Si) and carbon (C) in the form of particles were mixed ata molecular level was prepared (FIGS. 2 and 3 ). The silicon-carboncomposite had a diameter of about 200 nm and a density of 0.42 g/cc.

1-2. Manufacture of Electrode Plate

The amorphous silicon-carbon composite prepared in Example 1-1 was usedas a negative electrode active material. 80 wt. % of a negativeelectrode active material, 10 wt. % of binder (PAA/CMC, 1:1 weightratio), and 10 wt. % of conductive material (super-P) were dispersed inwater to prepare a negative electrode slurry, and applied to a copperelectrode to prepare an electrode plate.

1-3. Manufacture of Lithium Secondary Battery

The electrode plate prepared in Example 1-2 was used as a negativeelectrode. Lithium metal was used as a counter electrode, and apolyethylene separator was interposed between the negative electrode andthe counter electrode. A mixed solvent of ethylene carbonate and diethylcarbonate (EC/DEC, 3:7, volume ratio) using 1.3M LiPF₆ was used as anelectrolyte solution and 10 wt. % of FEC was used as an additive toprepare a coin cell.

Comparative Example 1

1-1. Preparation of Silicon-Carbon Composite

In order to adjust the discharging capacity to 600 mAh/g when evaluatingthe lifetime characteristics, about 15 wt. % of Si (theoreticalcapacity, about 3500 mAh/g) and about 85 wt. % of graphite (theoreticalcapacity, about 372 mAh/g) were simply mixed in the mortar to prepare asilicon-carbon composite (Si-Graphite) (FIG. 4 ).

1-2. Manufacture of Electrode Plate

An electrode plate was prepared in the same manner as in Example 1-2,except that the silicon-carbon composite (Si-Graphite) prepared inComparative Example 1-1 was used as a negative electrode activematerial.

1-3. Manufacture of Lithium Secondary Battery

A coin cell was prepared in the same manner as in Example 1-3, exceptthat the electrode plate prepared in Comparative Example 1-2 was used asa negative electrode.

Comparative Example 2

2-1. Preparation of Silicon-Oxygen-Carbon Composite

3 g of silicone oil was placed in an alumina container and heat treatedat 900° C. in an inert atmosphere furnace. Thereafter, after thetemperature inside the furnace was dropped to room temperature, asilicon-oxygen-carbon composite (SiOC) was obtained (FIG. 5 ).

2-2. Manufacture of Electrode Plate

The electrode plate was prepared in the same manner as in Example 1-2except that the silicon-oxygen-carbon composite (SiOC) prepared inComparative Example 2-1 was used as a negative electrode activematerial.

2-3. Manufacture of Lithium Secondary Battery

A coin cell was prepared in the same manner as in Example 1-3, exceptthat the electrode plate prepared in Comparative Example 2-2 was used asa negative electrode.

Comparative Example 3

3-1. Preparation of Amorphous Silicon-Carbon Composite

An amorphous silicon-carbon composite (Si—C) was prepared in the samemanner as in Example 1-1 except that the ball mill step was notperformed (FIGS. 6 and 7 ). The silicon-carbon composite has a diameterof about 3 μm and a density of 0.66 g/cc.

3-2. Manufacture of Electrode Plate

The electrode plate was prepared in the same manner as in Example 1-2except that the silicon-carbon composite (Si—C) prepared in ComparativeExample 3-1 was used as a negative electrode active material.

3-3. Manufacture of Lithium Secondary Battery

A coin cell was prepared in the same manner as in Example 1-3, exceptthat the electrode plate prepared in Comparative Example 3-2 was used asa negative electrode.

Experimental Example 1. Analysis of Crystal Structure of Composite

The XRD of the amorphous silicon-carbon composite (Si—C) prepared inExample 1-1, the silicon-carbon composite (Si-Graphite) prepared inComparative Example 1-1, and the silicon-oxygen-carbon composite (SiOC)prepared in Comparative Example 2-1 was measured (FIG. 8 ).

The silicon-carbon composite (Si—C) of Example 1-1 exhibited wide rangepeaks at 32 degrees and 60 degrees. Since the silicon of thesilicon-carbon composite (Si-Graphite) of Comparative Example 1-1 is anon-amorphous crystalline material, six silicon peaks (about 28 degrees,degrees, 56 degrees, 69 degrees, 76 degrees, and 88 degrees) wereclearly visible and graphite peaks were appeared at about 26, 35 and 44degrees. Also, the silicon-oxygen-carbon composite (SiOC) of ComparativeExample 2-1 showed wide range peaks at 30 degrees and 42 degrees.

Therefore, it was confirmed that the composite prepared in Example 1-1and Comparative Examples 1-1, and 2-1 had different materials andphysical properties.

Experimental Example 2. Evaluation of Electrical Conductivity ofElectrode Plate

Electrical conductivities of the electrode plates prepared in Example1-2, and Comparative Examples 1-2 and 2-2 were measured using afour-point probe (FIG. 9 ).

The electrode plate of Example 1-2 exhibited a low resistance valuebecause of its excellent electrical conductivity. The electrode plate ofComparative Example 1-2 showed low resistance value due to excellentelectrical conductivity, since the graphite has a form in which carbonlayers are layered one upon another, and thus has excellent electricalconductivity. However, the electrode plate of Comparative Example 2-2comprises silicon-oxygen-carbon composite (SiOC), which is a ceramicmaterial, and exhibits low electrical conductivity, which resulted invery high resistance.

Experimental Example 3. Evaluation of Battery Performance

Evaluation of initial charging/discharging, evaluation of life timecharacteristics, and evaluation of rate performance characteristics ofthe coin cells prepared in Example 1-3, and Comparative Examples 1-3,2-3, and 3-3 were performed.

(1) Evaluation of Initial Charging/Discharging

In order to observe charging/discharging characteristics depending onthe type of silicon composite, charging/discharging rates of the coincells prepared in Example 1-3, and Comparative Examples 1-3 and 2-3 werefixed at 0.05 C-rate, and the operating voltage was set to 0.005 to 2.5V, and then charging/discharging characteristics of the coin cells weremeasured (FIG. 10 ).

In all of the coin cells of Example 1-3, and Comparative Examples 1-3and 2-3, the capacity decreased as the C-rate increased, and the initialcapacity level was restored at the final 0.2 C-rate.

Also, in order to observe the characteristics of charging/dischargingdepending on the size of the diameter of the silicon composites,charging/discharging characteristics of the coin cells prepared inExample 1-3 and Comparative Example 3-3 were observed, and theexperiment was carried out in the same manner as described above (FIG.11 ).

The results are shown in Table 1 below.

TABLE 1 Charging Discharging Composite capacity capacity Efficiency size(mAh/g) (mAh/g) (%) Example 1-3 about 200 nm 1182 892.4 75.5 Comparative about 3 μm  927 579.4 62.5 Example 3-3

From the results shown in Table 1, it can be seen that the siliconcomposite of Example 1-3 having a small diameter has bettercharging/discharging characteristics than the silicon composite ofComparative Example 3-3 having a diameter exceeding 1 μm.

From this, it can be seen that the smaller the diameter of the siliconcomposite is, the better the charging/discharging effect is.

(2) Evaluation of Lifetime Characteristics

In order to observe the lifetime characteristics depending on the typeof silicon composite, the charging/discharging rate of the coin cellsprepared in Example 1-3, Comparative Examples 1-3 and 2-3 was fixed at0.2 C-rate, the operating voltage was set to 0.005 to 2.5 V, and then 50cycles were performed to measure the lifetime characteristics of thecoin cell. The results are shown in Table 2 and FIG. 12 below.

TABLE 2 Initial capacity Capacity after 50 (mAh/g) cycles (mAh/g)Example 1-3 1036  1033  Comparative Example 1-3 640 556 ComparativeExample 2-3 348 281

The coin cell of Example 1-3 resulted in excellent lifetimecharacteristics because the capacity of the cell was not substantiallyreduced even when the cycles progressed.

However, the coin cell of Comparative Example 1-3 has a very unstableconnection since silicon is point-contacted with graphite particles.Therefore, in the coin cell of Comparative Example 1-3, as the cycleprogressed, the volume expansion of silicon occurred, resulting in adecrease in the capacity of the cell and a poor lifetime characteristic.

Also, as the cycle of the coin cell of Comparative Example 2-3progressed, the capacity of the cell decreased and the lifetimecharacteristic was not good.

In addition, in order to observe the lifetime characteristics dependingon the size of the diameter of the silicon composites, the lifetimecharacteristics of the coin cells prepared in Example 1-3 andComparative Example 3-3 were observed and the experiment was carried outin the same manner as described above except that 100 cycles werecarried out (FIG. 13 ).

As a result, in Examples 1-3, capacity was saturated before 50 cyclesand stable lifetime characteristics were shown, but, Comparative Example3-3 did not saturate during 100 cycles and showed low capacity.

Therefore, it can be seen that the smaller the diameter of the siliconcomposite is, the better the lifetime characteristic is.

(3) Evaluation of Rate Performance

Charging and discharging of the coin cells prepared in Example 1-3, andComparative Examples 1-3 and 2-3 were carried out at a rate of 0.2C-rate for 20 cycles and 0.5 C-rate for 10 cycles, and then the rate ofcharging and discharging was controlled at 1, 2, 3, 4, 5, 6, 7, 8, 9,and 10 C-rate every 5 cycles, and finally, the rate was returned to 0.2C-rate to confirm whether the capacity level was restored back to theinitial capacity level (FIG. 14 ).

The cell capacity of all the coin cells of Example 1-3 and ComparativeExample 2-3 decreased as the cycles progressed, and the cell capacitywas restored to the initial capacity level at the last 0.2 C-rate.

However, in the case of the coin cell of Comparative Example 1-3,lithium ions are intercalated between the graphite layers, and themigration speed of lithium ions is very slow. Therefore, the coin cellof Comparative Example 1-3 showed a poor rate performance. In the caseof the coin cell of Example 1-3, since the lithium ion is stored in theform of an alloy, it was confirmed that even though the electricalconductivity is relatively lower than that of graphite, the rateperformance is excellent.

From the above results, it was confirmed that the amorphoussilicon-carbon composites with diameters of 10 nm to 1 μm, which iscomposed of silicon and carbon mixed at a molecular level, exhibitexcellent electrical conductivity, charging/discharging characteristics,and lifetime characteristics.

The invention claimed is:
 1. An amorphous silicon-carbon composite,comprising: silicon and carbon mixed at a molecular level, wherein theamorphous silicon-carbon composite is in the form of particles having adiameter of 10 nm to 1 μm, wherein the amorphous silicon-carboncomposite is formed by a pyrolysis deposition process for a siliconsource and a carbon source, wherein the amorphous silicon-carboncomposite comprises a silicon-carbon covalent bond, a silicon-siliconcovalent bond, and a carbon-carbon covalent bond, and wherein thesilicon-carbon covalent bond, the silicon-silicon covalent bond, and thecarbon-carbon covalent bond are irregularly present in the amorphoussilicon-carbon composite.
 2. The amorphous silicon-carbon composite ofclaim 1, wherein the amorphous silicon-carbon composite furthercomprises a heteroatom, and further comprises at least one bond selectedfrom the group consisting of a heteroatom-carbon covalent bond and aheteroatom-silicon covalent bond.
 3. The amorphous silicon-carboncomposite of claim 2, wherein the heteroatom comprises at least oneselected from the group consisting of boron, phosphorus, nitrogen, andsulfur.
 4. The amorphous silicon-carbon composite of claim 1, whereinthe amorphous silicon-carbon composite comprises silicon and carbon in aweight ratio of 3:7 to 7:3.
 5. The amorphous silicon-carbon composite ofclaim 1, wherein the amorphous silicon-carbon composite has a density of0.2 g/cc to 0.6 g/cc.
 6. A negative electrode for a lithium secondarybattery comprising an active material; a conductive material; and abinder, wherein the active material comprises the amorphoussilicon-carbon composite of claim
 1. 7. A lithium secondary batterycomprising a positive electrode; a negative electrode; a separatorinterposed between the positive electrode and the negative electrode;and an electrolyte solution, wherein the negative electrode is thenegative electrode of claim 6.